In recent years, high-strength steel sheets having a TS (tensile strength) of 980 MPa or more and a small thickness have been actively used for structural parts of automobiles for the purpose of ensuring the crash safety of occupants and for the purpose of improving fuel efficiency by automotive lightening. However, the increase in strength of steel sheets usually leads to a reduction in stretch flangeability or bendability of the steel sheets. Therefore, there are increasing demands for steel sheets having high strength and excellent formability. Furthermore, the reduction in ductility of steel sheets due to the increase in strength thereof impairs plastic deformability during crashes and therefore leads to the reduction of energy absorption. Hence, an improvement in energy absorption during high-speed deformation is desired.
To cope with such demands, for example, Japanese Unexamined Patent Application Publication No. 9-13147 discloses a high-strength galvannealed steel sheet which has a TS of 800 MPa or more, excellent formability, and excellent coating adhesion and which includes a galvannealed layer disposed on a steel sheet containing 0.04% to 0.1% C, 0.4% to 2.0% Si, 1.5% to 3.0% Mn, 0.0005% to 0.005% B, 0.1% or less P, more than 4N to 0.05% Ti, and 0.1% or less Nb on a mass basis, the remainder being Fe and unavoidable impurities. The content of Fe in the galvannealed layer is 5% to 25%. The steel sheet has a multi-phase microstructure containing a ferritic phase and a martensitic phase.
Japanese Unexamined Patent Application Publication No. 11-279691 discloses a high-strength galvannealed steel sheet having good formability. The galvannealed steel sheet contains 0.05% to 0.15% C, 0.3% to 1.5% Si, 1.5% to 2.8% Mn, 0.03% or less P, 0.02% or less S, 0.005% to 0.5% Al, and 0.0060% or less N on a mass basis, the remainder being Fe and unavoidable impurities; satisfies the inequalities (Mn %)/(C %) ≥15 and (Si %)/(C %)≥4; and has a ferritic phase containing 3% to 20% by volume of a martensitic phase and a retained austenitic phase.
Japanese Unexamined Patent Application Publication No. 2002-69574 discloses a high-strength cold-rolled steel sheet and high-strength plated steel sheet having excellent stretch flangeability and low yield ratio. The high-strength cold-rolled steel sheet and the high-strength plated steel sheet contain 0.04% to 0.14% C, 0.4% to 2.2% Si, 1.2% to 2.4% Mn, 0.02% or less P, 0.01% or less S, 0.002% to 0.5% Al, 0.005% to 0.1% Ti, and 0.006% or less N on a mass basis, the remainder being Fe and unavoidable impurities; satisfy the inequality (Ti %)/(S %)≥5; have a martensite and retained austenite volume fraction of 6% or more; and satisfy the inequality α≤50000×{(Ti %)/48+(Nb %)/93+(Mo %)/96+(V %)/51}, where α is the volume fraction of a hard phase structure including a martensitic phase, a retained austenitic phase, and a bainitic phase.
Japanese Unexamined Patent Application Publication No. 2003-55751 discloses a high-strength galvanized steel sheet having excellent coating adhesion and elongation during molding. The high-strength galvanized steel sheet includes a plating layer disposed on a steel sheet containing 0.001% to 0.3% C, 0.01% to 2.5% Si, 0.01% to 3% Mn, and 0.001% to 4% Al on a mass basis, the remainder being Fe and unavoidable impurities, and which contains 0.001% to 0.5% Al and 0.001% to 2% Mn on a mass basis, the remainder being Zn and unavoidable impurities, and satisfies the inequality 0≤3−(X+Y/10+Z/3)−12.5×(A−B), where X is the Si content of the steel sheet, Y is the Mn content of the steel sheet, Z is the Al content of the steel sheet, A is the Al content of the plating layer, and B is the Mn content of the plating layer on a mass percent basis. The steel sheet has a microstructure containing a ferritic primary phase having a volume fraction of 70% to 97% and an average grain diameter of 20 μm or less and a secondary phase such as an austenitic phase and/or a martensitic phase, having a volume fraction of 3% to 30% and an average grain diameter of 10 μm or less.
However, for the high-strength cold-rolled steel sheets and the high-strength galvanized steel sheets disclosed in JP '147, JP '691, JP '574 and JP '751, excellent formability including stretch flangeability cannot be achieved if attempts are made to achieve a TS of 980 MPa or more. Furthermore, energy absorption during crash is not taken into account.
It could therefore be helpful to provide a high-strength galvanized steel sheet having a TS of 980 MPa or more, excellent formability including stretch flangeability, and excellent energy absorption (crashworthiness) during high-speed deformation and a method of manufacturing the same.